Rubber articles vulcanized using peroxides instead of sulfur as crosslinking agents possess certain advantageous properties such as superior heat resistance, compression set and electrical insulation for molded articles, and increased stability in compounding with colored substances.
Vulcanization with peroxides, however, is hindered by oxygen. If vulcanization with peroxides is carried out in the presence of air, crosslinking can proceed inadequately on the surface of the rubber article. Inadequate crosslinking can lead to surface defects such as unacceptable tackiness, reduction in strength, or insufficient hardness which can result in performance problems of the rubber article such as poor wear resistance, mar resistance, and slipperiness.
As a result, vulcanization of rubber with peroxides has generally been conducted in environments shielded from air such as in process for press molding, injection molding, or transfer molding. For fabricating hoses and electric wires using an autoclave cure, air has been replaced with steam.
Methods have been described in the art for preventing the tackiness of surfaces of peroxide-cured rubber articles exposed to air during vulcanization. Many of these methods require subjecting the article to surface-treatment compositions, which impart a non-tacky and water-repellent film on the surface of the article. Surface treatment of peroxide-cured EPDM articles is described, for example, in European Patent Application 482,480 A1 and U.S. Pat. No. 4,491,653.
U.S. Pat. No. 5,213,723 describes a method for preparing a peroxide-cured rubber product in the presence of oxygen using electron beams to irradiate the cured rubber product. U.S. Pat. No. 4,983,685 describes a method for preparing a peroxide-cured rubber product in the presence of oxygen by incorporating an antioxidant into the rubber product prior to curing.
It is an object of this invention to provide an EPDM rubber composition which can be cured with organic peroxides in the presence of oxygen to produce an EPDM rubber product that exhibits a tack-free surface.